The present invention relates to a method for manufacturing a gradiometer having a three-dimensional structure for a single or multi-channel device for measuring magnetic fields with field intensities down to below 10.sup.-10 T and particularly below 10.sup.-12 T, in which the superconducting gradiometer coils of predetermined dimensions which lie in different planes are connected to each other via superconducting connecting lines and are coupled to at least one superconducting quantum interference element (SQUID). A prior art method of this type is known from DE-OS No. 32 47 543.
The use of superconducting quantum interference elements which are generally called "SQUIDs" (abbreviation for "Superconducting Quantum Interference Devices") for measuring very weak magnetic fields is generally known (J. Phys. E.: "Sci. Instrum.", Vol. 13, 1980, pages 801 to 813; "IEEE Transactions on Electron Devices", Vol. Ed.-27, No. 10, October 1980, pages 1896-1908). As a preferred field of application for these elements is therefore also considered medical technology, especially magnetocardiography and magnetoencephalography, where magnetic heart or brain waves with field strengths on the order of magnitude of 50 pT and 0.1 pT respectively occur ("Biomagnetism-Proceedings of the Third International Workshop on Biomagnetism, Berlin 1980", Berlin/New York 1981, pages 3 to 31; "Review of Scientific Instruments", Vol. 53, No. 12, December 1982, pages 1815 to 1845).
A device for measuring such biomagnetic fields contains essentially the following components:
1. A SQUID as the field-sensor proper with a so-called gradiometer,
2. A flux transformer in the form of a coil arrangement for coupling the field to be examined into the SQUID,
3. Electronic equipments for picking up and processing signals,
4. Shields for the earth's magnetic field and external interference fields, and
5. A cryo system for assuring superconductivity of the sensor and the gradiometers.
The design and operation of such single-channel devices are known. In these devices, the magnetic field to be detected, which is up to 6 orders of magnitude smaller than external interference fields, is generally coupled inductively via a three-dimensional coil arrangement into the circuit formed by an RF SQUID with a Josephson contact. Through combinations of a sensor coil, also called a detection coil, with one or more compensating coils, coil systems called first or higher-order gradiometers are realized. By appropriate manual adjustment with such gradiometers, the three components of a magnetic field which is homogeneous in the region of the coils or also its content of homogeneous gradients can be largely suppressed and the biomagnetic near field which is still heavily non-uniform in the vicinity of the gradiometers can be picked up selectively.
In order to obtain with such a device a three-dimensional field distribution, measurements must be made sequentially at different points of the region to be examined. The difficulty arises that the coherence of the field data over the measuring time required therefore is no longer assured, and in addition, clinically insufferable measuring times result. It has therefore been proposed to make a multi-channel measurement instead of the known single channel measurement (see, for instance, "Physica", Vol. 107B, 1981, pages 29 and 30). Besides an RF SQUID, each channel comprises a tunable superconducting gradiometer, the coils of which are coupled to the SQUID via connecting lines and a coupling coil, also called a coupling transformer. In such a device, however, a considerable time-consuming effort results with respect to the tuning of the individual channels, since in this device- the gradiometer on the one hand and the SQUID with its coupling coil on the other hand are each arranged on a support body of their own, where these parts can be connected to each other via detachable connecting lines. With such a connecting technique, however, constant tuning of the respective flux transformer cannot be assured from the start. Rather, an adjustment of all channels is required prior to each measurement, which also influence each other. In addition, mutual interference of the RF circuits is unavoidable in such an arrangement. While the mutual interference of the channels in an adjacent arrangement as well as the intrinsic noise of the individual channels can be reduced by the use of d-c SQUIDs instead of RF SQUIDs (see, for instance, "IEEE Transactions on Magnetics", Vol. MAG-19, No. 3, May 1983, Pages 835 to 844), the adjustment of the individual channels of a corresponding multichannel gradiometer system of modular design is difficult to control.
The three-dimensionally structured gradiometers of the known devices are generally made of superconducting wire on appropriate coil forms where it is hardly possible for the adjustment tolerances due to the manufacture to fall below approximately 10.sup.-3. An improvement of these tolerances is achieved by a subsequent mechanical adjustment. However, with this method, a realization of complex rows of gradiometers, also called gradiometer arrays, such as are required for multichannel measuring devices can be achieved only with difficulty, since a mechanical adjustment can practically not be carried out in such devices.
From DE-OS No. 32 47 543 cited above, it is furthermore known to fabricate such gradiometer arrays by a thin-film planar technique. According to this known method, the coils associated with each gradiometer are first applied by the mentioned technique in different planes of a three-dimensional substrate body. These coils must then be interlinked with each other and the SQUID(s) via separate connecting lines (FIGS. 5 to 7). While the known thin-film technique permits a better adjustment as well as the realization of more complex structures, the superconducting connecting technique required therefor is very costly.